1. Field of the Invention
This invention relates to low power communication network technology and, more particularly, to a low power communication system and method for communicating in the low power communication network.
2. Description of the Related Art
U.S. Pub. No. 2005/0068928 AI discloses a conventional system and method for low power communication in a low power communication network.
In a low power communication network, such as a wireless sensor network (WSN), an individual network node is in a wake-up state (i.e. a transmitting state or receiving state, referred to also as an activated state) only for a short period of time, and is in a sleep state during the rest of time. In the sleep state, the network node only needs to keep the fundamental functional modules in an operational state (such as a wake-up timer), thus, the power consumption of the network node is merely the power needed in supporting the fundamental functional modules.
FIG. 1 shows a typical superframe (SF) structure in a low power communication network. Such a frame structure comprises a beacon frame, a wake-up period (data period) and a sleep period, wherein the wake-up period is also referred to as a data period, and the length of time of the superframe is represented by tsF. A first network node as a data collecting end and a second network node as the data transmitting end communicate based on the superframe structure as shown in FIG. 1. As to the second network node, after it detects the beacon frame transmitted by the first network node, the second network node changes into the wake-up state from the sleep state (namely, enters the wake-up state from the sleep state) and starts to transmit first network node. After the data transmission, the second network node changes again into the sleep state (namely, enters the sleep state from the wake-up state) until it receives a next beacon frame and changes back again into the wake-up state to transmit the data. For this reason, in order to carry out data transmission with its neighboring network nodes, the network node must synchronize its wake-up period with that of the network node as its communication counterpart to ensure the successful communication between the two network nodes. The beacon frame is thus used to indicate the position of the wake-up period and also to synchronize the wake-up periods of the two network nodes.
In the low power communication network with a star topological structure as shown in FIG. 2, a network coordinator 100, here a network gateway, carries out communication with each of the network nodes, such as a network node 101, within the area covered by the network coordinator 100. The network coordinator transmits beacon frames periodically depending on its length of time of its superframe. Each of the network nodes in the area covered by the network coordinator synchronizes the data transmission between the network node and the network coordinator by scanning the beacon frames, i.e. synchronizes the wake-up period of the network node with the network coordinator. Besides being a network gateway to other networks, the network coordinator can also be a data source or a sink for data to be processed, and furthermore, can be a router for data exchange between network nodes.
It is known from U.S. Pub. No. 2005/0068928 A1 that the network nodes may passively scan, i.e., simply listen for beacon frames or, alternatively, send a probe request to the access point which, in turn, sends back a probe response in the form of a dummy beacon frame indicating the transmission time of the beacon frame (active scan). In the case where many, e.g., fifty, network nodes are being served by a single access point, a large number of probe requests may be generated, which may slow down the network and generate an unnecessary drain of power within the network nodes. Thus, in order to help conserve battery power and network resources, gratuitous probe responses (GPR) are interjected between the beacon frames to provide the network nodes with needed information in a more schedulable manner such that any individual network node may be powered up into an active mode to receive a beacon frame or a GPR at a target beacon transmission time or a target GPR transmission time.
Another description of the passive and active scanning is provided by Mike Baker et al: “Measurement Collection for Neighbor Tracking and Handover in an IEEE 802.11 Home Environment”, 6th World Wireless Congress, May 2005, pages 196-201.
EP 1 548 984 A1 discloses a base station transmitting a main beacon indicating that all network nodes are allowed to perform transmission, and additionally transmitting a sub beacon (dummy beacon) in response to the successful reception of a data packet from a network node, where the dummy beacon includes the transmitting time of the main beacon. Thus, the network node may divide the next data packet if it is longer than the remaining time until the main beacon so that data transmission is finished before transmission of the main beacon is started, and thereby overlap of transmission timings of the beacon and the transmission data are prevented and the beacon interval can be fixed.
WO 2006/138058 A2 discloses transmitting full parent beacon frames periodically at a beacon frame rate for a particular network identifier from an access point. The access point further transmits mini-beacon frames in between consecutive full parent beacon frames for the particular network identifier, where each mini-beacon frame has a subset of information in the full parent beacon frame. The subset of information includes timing information on the next parent beacon frame, so that a network node associated to the access point can determine when the next full parent beacon frame is to be transmitted. The known method is particularly useful for an access point that supports multiple basic service set identifiers (BSSIDs), i.e., that can act as multiple virtual access points. As beacon frames can be quite large for multiple BSSID, the known method avoids the potential waste of bandwidth that would occur if only full-size beacons were broadcasted by the access point.
FIG. 3 is a schematic diagram of a situation of a mobile network gateway with a star topology moving in a network. Assume that a mobile network gateway (such as a mobile data collection device in WSN) passes through an area distributed with a plurality of wireless nodes (such as the sensing nodes in WSN, which are typically disposed in the network fixedly), and intends to communicate with all the wireless nodes in the area covered by the mobile network gateway. As shown in FIG. 3, a mobile network gateway moves from a first position to a second position, thus, the area covered by the mobile network gateway is changed from area 1 to area 2. In this case, the wireless nodes in area 1 include wireless nodes 1, 2, 3, 4, 5 and 6, and the wireless nodes in area 2 include wireless nodes 2, 3, 4, 5, 7, 8 and 9, with each wireless node represented by a single circle. The above-described beacon frame assisted wake-up synchronization method is not applicable to such a situation. Since the wireless nodes outside the area covered by the mobile network gateway cannot receive the beacon frame, these wireless nodes cannot be synchronized, and therefore cannot be in the sleep state. Furthermore, due to the mobility of the mobile network gateway, the wireless nodes entered into the area covered by the mobile network gateway have to monitor consecutively the communication link to detect at the appropriate time the beacon frame transmitted by the mobile network gateway, and therefore, they cannot be in the sleep state. As a consequence, the wireless nodes, the wake-up periods of which cannot be synchronized with that of the mobile network gateway, will have power consumptions of a much greater level than those the wake-up periods of which have been synchronized with that of the mobile network gateway. As for a typical low power network application situation in which wireless node devices are powered by batteries, this will cause not only a device maintenance period being too short (for example, the device needs to be charged too frequently), but also the wake-up periods between the mobile network gateway and the wireless nodes being unable to be synchronized promptly and efficiently.